Method for manufacturing a sensor

ABSTRACT

In a sensor and a method for manufacturing a sensor, a movable element is patterned out of a silicon layer and is secured to a substrate. The conducting layer is subdivided into various regions, which are electrically insulated from one another. The electrical connection between the various regions of the silicon layer is established by a conducting layer, which is arranged between a first and a second insulating layer.

This application is a division of prior application No. 08/718,603 filedSep. 24, 1996, now U.S. Pat. No. 5,756,901.

BACKGROUND INFORMATION

A sensor and a method for manufacturing a sensor are described in GermanPatent Application No. 43 18 466, where a substrate is provided with asilicon layer. A movable element for the sensor is patterned out of thesilicon layer.

SUMMARY OF THE INVENTION

An advantage of the sensor according to the present invention and of themethod according to the present invention is that an especiallyadvantageous contacting of the sensor elements can be ensured throughthe conducting layer. The conducting layer is insulated by an especiallyhigh-grade dielectric insulation from an undesired electrical contactwith the silicon layer.

Because of the adapted thermal expansion, silicon is especially suitedas a substrate material, because it makes it possible to avoid thermallyproduced strains, which can affect the characteristic of the sensor. Acomplete dielectric insulation of the printed conductors from theenvironment can be effected by using a first and second insulatinglayer. The movable element can then be completely surrounded by a frame,so that a good separation can be achieved between the region in whichthe movable element is arranged and contact regions. The movable elementcan then be hermetically separated by a cover from the environment. Whenthe movable element is displaceable in response to an acceleration, thesensor element can be used as an acceleration sensor. A large usefulsignal can then be attained when a plurality of fixed and movableelectrodes are used. The ability to measure the sensor signal isimproved through the symmetrical arrangement of these electrodes. Themethod according to the present invention makes it possible to simplymanufacture a sensor element, only process steps being used, which arewell known from semiconductor technology. Furthermore, the methodaccording to the present invention requires only few lithographic steps.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 through 8 illustrate the various steps of the manufacturingmethod according to the present invention.

FIG. 9 shows a view of the silicon layer of an acceleration sensor.

FIG. 10 shows the arrangement of the conducting layer of the sensoraccording to FIG. 9.

FIG. 11 shows another exemplary embodiment of the sensor.

FIG. 12 shows yet another exemplary embodiment of the sensor.

DETAILED DESCRIPTION

FIG. 1 depicts a silicon substrate (10), on which is deposited a firstinsulating layer (1) and thereupon, a conducting layer (3). The firstinsulating layer (1) is a thermal oxide having a thickness of about 2.5micrometers, and a conducting layer (3) deposited thereon of polysiliconhas a thickness of about 0.5 micrometers. However, other layer materialsare also conceivable, e.g., insulating layer (1) can also consist ofother oxides, silicon nitride, or other insulating layers. Besidespolysilicon, metallic layers are also suitable for conducting layer (3),materials, such as tungsten, being selected which are not critical forthe subsequent high-temperature steps. The conducting layer (3), whichconsists here of polysilicon, is doped by means of doping out of the gasphase (POCl₃), the aim being to achieve a greatest possibleconductivity. All other processes for producing an amply dopedpolysilicon layer may be used as well.

A patterning of conducting layer (3), as shown in FIG. 2, then followsthrough a photolithographic process. The conducting layer (3) is therebysubdivided into individual regions, which are insulated from one anotherand can be used, e.g., as printed conductors or electrodes.

A second insulating layer (2) is deposited on the substrate according toFIG. 2. To deposit this layer, the deposition processes known fromsemiconductor technology for depositing dielectric layers can be used.Thus, besides silicon dioxide, silicon nitride, different glasses orother ceramic layers can also be deposited. For the further description,one will start out from the assumption that the first dielectric layer(1) consists of silicon oxide, which is formed from the thermaloxidation of the silicon substrate (10). The second dielectric layer (2)likewise consists of silicon oxide, which is produced, however, from thegas phase, e.g., through decomposition of silane.

One should note in this case that the thermal silicon oxide layer (1)has a greater density than the silicon oxide layer (2) deposited out ofthe gas phase. Given a chemical etching of the two layers, this leads tothe upper silicon oxide layer (2) etching more quickly than the bottomsilicon oxide layer (1). The top insulating layer (2) is then patternedin a photolithographic process, contact holes (4) being introduced intothe upper insulating layer (2) to allow contacting of the underlyingconducting layer (3). When printed conductors are patterned out ofconducting layer (3), they can be contacted through contact holes (4).

A polysilicon layer (5) is then deposited onto the surface of thesubstrate according to FIG. 3. The polysilicon starting layer (5) coversthe surface of the second insulating layer (2) and serves as a nucleusfor the subsequent deposition. A suitable doping process, e.g.implantation or driving in dopants out of the gas phase, is used toensure that the polysilicon starting layer (5) is heavily doped. Allcustomary methods used in semiconductor technology for depositingpolysilicon layers on to dielectric layers are suitable for depositingthe polysilicon starting layer.

The thick silicon layer is then deposited in a further process step.This deposition takes place in an epitaxial reactor. An epitaxialreactor of this type is a system for depositing silicon layers, whichare used in semiconductor technology for producing monocrystallinesilicon layers on a monocrystalline silicon substrate. The deposition oflayers of this type takes place, as a rule, at temperatures of more than1000 degrees celsius and, thus, layers in the order of magnitude of afew 10 micrometers can be attained.

Since in the present process, the deposition in the epitaxial reactordoes not take place on a monocrystalline silicon substrate, but ratheron the polycrystalline silicon starting layer, no monocrystallinesilicon forms, but rather a thick polycrystalline silicon layer (6),which is referred to in the following as a thick Si-layer (6). Thecrystalline properties of the thick silicon layer (6) are able to beinfluenced by the deposition conditions under which the polycrystallinesilicon starting layer (5) is produced. In addition, the heavy doping ofthe polysilicon starting layer (5) effects a doping of the thick siliconlayer (6) starting from the bottom side. Furthermore, during the timethat the thick silicon layer (6) grows and in a subsequent dopingprocess, a further doping of the thick silicon layer (6) takes place.The subsequent doping of the thick silicon layer (6) can follow, inturn, by means of implantation, doping out of the gas phase or by meansof any other doping process known from semiconductor technology. In thisprocess, the polysilicon starting layer (5) becomes a part of the thicklayer (6). In the area of contact holes (4), the thick layer (6) has adirect contact with conducting layer (3).

A patterned metal layer (7) is then still applied to the top side ofthick layer (6). The metal layer can be applied, e.g., over the entiresurface and subsequently patterned.

A patterning of the thick Si-layer (6), as shown in FIG. (6), thenfollows in a further photolithographic process. For this purpose, amask, e.g. a photomask, is applied to the top side of layer (6) and alsoprotects metal layer (7) in the subsequent etching process. Adry-etching (plasma etching), for example, of the thick Si-layer (6)then follows through openings in the photoresist mask, trenches (9)being introduced thereby. Trenches (9) having a high aspect ratio, i.e.,a substantial depth and small lateral dimensions, can then be producedthrough a plasma etching process.

Following the deposition, the thick Si-layer (6) initially has arelatively rough surface. A photo-resist is applied to planarize thissurface roughness, and an etching process follows, e.g., in an SF₆ /O₂plasma, which etches resist and polysilicon at the same etching rate.The photoresist is first applied in a liquid state and forms a flatsurface, so that the surface roughness of the layer (6) is diminished.

The trenches (9) extend from the top side of the thick Si-layer (6) downto the second insulating layer (2). The layer (6) is subdivided intoindividual regions, which are isolated from another, to the extent thatthey are not interconnected via conducting layer (3).

An etching medium is then introduced to the second insulating layer (2)through trenches (9), the etching medium effecting an etching of theelectrical layer (2). The insulating layer (2) is then etched startingfrom trenches (9), an etching not only taking place directly undertrenches (9), but a lateral undercut-etching also taking place underlayer (6) in dependence upon the etching duration, as shown in FIG. 7.The undercut-etching underneath silicon layer (6) will be described ingreater detail with respect to FIG. 9. FIG. 7 illustrates the state thatexists following the etching of the insulating layer (2). Of course,insulating layer (1) is also attacked, however minimally, due to thegreater density of the material and the short etching duration. However,to the extent that it is desired for the functioning of the manufacturedsensor element, an etching of the lower insulating layer (1) can followas the result of a longer etching duration.

FIG. 7 now shows an exemplary cross-section through a sensor element.Different functional regions are now patterned out of layer (6). Aconnection (terminal) region (20), which is completely surrounded bytrenches (9), is patterned out from underneath the metallization. Thisconnection region (20) is, thus, completely isolated by trenches (9)from the rest of layer (6). However, since connection region (20) is indirect contact with conducting layer (3), a contact to other regions oflayer (6) can be established through conducting layer (3), which is thendesigned in this case as a printed conductor.

The purpose of connection region (20) with its applied metallization (7)is to secure bonding wires, which are used to establish an electricalcontact to the sensor structure. The conducting layer (3), which isdesigned as a printed conductor, permits an electrical connectionbetween connection region (20) and an anchoring region (22). Theanchoring region (22) is likewise built up on conducting layer (3) andis in electrical contact with conducting layer (3). In addition, as aresult of this connection, the anchoring region (22) is permanentlyanchored to the substrate.

The anchoring region (22) goes over to changes into a free-standingregion (23). The free-standing region (23) is characterized by theremoval of insulating layer (2) underneath free-standing region (23).Therefore, free-standing region (23) can be moved toward the substrate(10).

In addition, a frame (21), which is likewise completely surrounded bytrenches (9), is shown in FIG. 7. The trenches (9) isolate frame (21)from the other regions of the Si-layer (6). In addition, frame (21) isconstructed on the insulating layer and is, thus, insulated fromconducting layer (3) shown in FIG. 7.

The functioning of frame (21) is elucidated in FIG. 8. In the furthercourse of the manufacturing process, a cover (13), which hermeticallyseals the sensor, is secured to frame (21). Thus, a hermetically tightsealing of the sensor element is achieved. To establish the connectionbetween frame (21) and cover (13), solder glass layer (8) isreflow-soldered on. The solder glass layer can then be deposited, e.g.,by means of silk-screen printing on to the cover. When not just onesensor element, but instead several sensor elements are fabricatedconcurrently on substrate (10), then a cover plate (14) with a pluralityof corresponding covers (13) can be provided. The substrate plate (10)having the plurality of sensor elements and the cover plate having theplurality of corresponding covers (13) are then joined together, and theindividual sensor elements are diced into individual sensors alongdicing lines (15). The costs for each individual sensor element are keptlow by manufacturing several sensors concurrently.

FIG. 9 illustrates a plan view of an acceleration sensor, which isfabricated according to the method of FIGS. 1 through 8. For the sake ofclarity, three electrode sets are shown as examples. In an actualcomponent, the number can be clearly higher. The plan view of FIG. 9thereby corresponds to a plan view of the structure, as it appears inthe top view according to the manufacturing steps of FIGS. 6 and 7,metal layer (7) not being shown, however, for the sake of simplicity.Thus, the plan view of Figure (9) corresponds to a plan view of thepatterned, thick silicon layer (6). However, FIGS. 6 and 7 do notrepresent a cross-section through the structure shown in FIG. 9, butrather merely a simplified illustration of all essential elements, asthey are depicted in the case of the acceleration sensor of FIG. 9.

As FIG. 9 reveals, frame (21) is designed as a square frame, whichcompletely surrounds one region. The acceleration-sensitive element isarranged inside the frame, the acceleration-sensitive element havingregions which are permanently joined to the substrate, and regions whichare detached from the substrate. Permanently joined to the substrate arethe anchoring regions (22) and the connecting elements (25). Separatedfrom the substrate are pressure bar (30), flexural elements (31),seismic mass (32), movable electrodes (33), and fixed electrodes 34,35). The undercut-etching underneath layer (6) takes place duringfabrication in a time-limited etching step. The etching is limited intime so as to undercut structures having a small lateral extent, whilestructures having a large lateral extent are not undercut.

The anchoring regions (22) and connecting elements (25) each have largelateral dimensions, so that the insulating layers 1, 2 situatedunderneath these structures are not completely undercut. The flexuralelements, the movable electrodes, and the fixed electrodes have onlysmall lateral dimensions, so that the insulating layers under thesestructures are quickly undercut. The seismic mass (32) and pressure bars(30) are, in fact, designed to be relatively large, however, they have aplurality of etching holes (36), so that pressure bars (30) and seismicmass (32) only have small lateral dimensions. The etching holes (36)each extend from the top side of Si-layer (6) to the bottom side of thethick silicon layer (6) and, thus, allow unhindered access of a mediumto subjacent insulating layers 1, 2, which serve as sacrificial layers.

The limitations with respect to the lateral dimensions do not apply tocontact holes, as denoted by reference numeral 4 in FIG. 7 or referencenumerals 40 through 49 in FIG. 9, since in the case of these contactholes, Si-layer 6 is directly connected to conducting layer 3. In thiscase, there cannot be any undercutting underneath layer 2, so that givenlarge enough lateral dimensions of conducting layer 3 in this region,structures with very small lateral dimensions can also be realized inthe thick Si-layer 6 situated above it. The lateral dimensions in theconducting layer 3 can thereby be correspondingly smaller, because thebottom insulating layer 1 is not as etchable.

To represent the electrical interconnections of the individual sensorcomponents, contact holes (40 through 49) are also shown in FIG. 9, butare not visible in the plan view of the layer (6). The contact holes ofFIG. 9 are once again shown separately in FIG. 10, and theinterconnection of the contact holes, which is established by theconducting layer (3), is shown. As FIG. 9 reveals, each movableelectrode (33) is arranged between two fixed electrodes. The fixedelectrodes (34) are each arranged in the negative Y-direction and thefixed electrodes (35) in the positive Y-direction of the correspondingmovable electrode (33). Three contact holes (40) are shown in FIG. 9,through which an electrical contact to three fixed electrodes (34) isestablished on the left side of the sensor element shown in FIG. 9.

As FIG. 10 reveals, the contact holes (40) are connected through theconducting layer (3), which is patterned as a printed conductor, to oneanother and to another contact hole (41). The contact hole (41)establishes a contact to a connecting element (25), which establishes anelectrical connection between the left side of the sensor element andthe right side of the sensor element. On the right side, this connectingelement (25) is then likewise electrically connected to the fixedelectrodes (34) and to another contact hole (46). As shown in FIG. 10,the contact hole (46) is connected through the conducting layer (3)patterned as a printed conductor to another contact hole (49), whichestablishes the electrical contact to a connection region (20) situatedoutside of frame (21). Thus, an electrical contact is established to allfixed electrodes (34) through this connection region (20), so that acapacitive signal can be tapped off from these electrodes.

Provided, accordingly, on the right side of the sensor element arecontact holes (42), which are connected through a printed conductor to acontact hole (43). The contact holes (42) establish, in turn, a contactto fixed electrodes (35). By way of a connecting element (25), a contactis then established to the fixed electrode (35) situated on the leftside and to a contact hole (44). As FIG. 10 reveals, through contacthole (44) and the printed conductor patterned out of conducting layer(3), a contact to a contact hole (47) and to a corresponding connectionregion is established. Thus, the signal from the fixed electrodes (35)can be measured at this connection region. An electrical contact tomovable electrodes (33) is established through the contact hole (45) viapressure bar (30), flexural elements (31), and seismic mass (32). Thecontact hole (45) is connected through a printed conductor formed out ofconducting layer (3) to a contact hole (48) and to a correspondingconnecting region (20) where the capacitive signal of the movableelectrodes is, thus, able to be tapped off.

When an acceleration occurs in the positive or negative Y-direction,then seismic mass (32) and movable electrodes (33) suspended thereon areshifted in the positive or negative Y-direction, since this structure isonly suspended on thin flexural elements (31). Because of their thinform, these flexural elements are able to be easily deformed in theY-direction. The deflection changes the distance of movable electrodes(33) relative to fixed electrodes (34) and (35) changes. Thus, thedeflection can be verified by measuring the capacitance between thefixed and movable electrodes.

FIG. 9 shows a plan view of Si-layer (6), metallization (7) not beingdepicted. A metal layer (7), which permits a bonding wire to beattached, is provided on each connection region (20). A solder glasslayer (8), which permits a cover (13) to be attached, is provided onframe (21).

As revealed in the cross-section through FIG. 7, frame (21) iscompletely isolated from the printed conductors formed from conductinglayer (3). In addition, as revealed in FIG. 10, the individual printedconductors formed out of conducting layer (3) are isolated from oneanother.

The pressure bar (30) is detached from the substrate and can expand orcontract in relation to the substrate. As a result, thermally producedstresses and/or stresses which arise in the process of fabricating thethick Si-layer (6), do not have any effect on the stress state offlexural elements (31). These stresses are compensated by the parallelarranged pressure bar (30).

The thickness of the first and second insulating layer 1, 2 lies in theorder of magnitude of a few micrometers; the layer of conducting layer(3), as a rule, under one micrometer; and the thickness of silicon layer(6) amounts to an order of magnitude of a few 10 micrometers. Sinceconducting layer (3) and layer (6) are separated in some regions, e.g.,underneath frame (21), only by the second insulating layer (2), which isa few micrometers thick, comparatively large parasitic capacitancesoccur. Therefore, the design of the sensor shown in FIG. 9 is soselected that the capacitances for both groups of fixed electrodes (34)and (35) are more or less equal. Furthermore, as a rule, conductinglayer (3) has a comparatively poorer conductivity than the thickSi-layer (6). Therefore, the design is selected so that of the six fixedelectrodes (34) in the first group, three are linked through aconducting layer (3) and three other electrodes through layer (6). Thisapplies correspondingly to a second group of fixed electrodes (35).Therefore, the parasitic lead resistances to the individual electrodesare more or less the same for both groups of fixed electrodes (34, 35).

FIG. 11 shows another example of a sensor according to the presentinvention, the cross-section through FIG. 11 essentially correspondingto the cross-section through FIG. 7, the identical reference numerals ofFIG. 11 denoting the same objects as the corresponding referencenumerals of FIG. 7. However, in contrast to FIG. 7, a vertical electrode(60), which is likewise patterned on conducting layer (3), is providedunderneath the etched-free region (23). By means of the verticalelectrode, for example, movements of the etched-free region (23) can beverified in a direction normal to the substrate. Furthermore, a verticalelectrode (60) of this type can be used as a shield electrode, whichshields the actual sensor structure from environmental influences.

A cross-section through another exemplary embodiment of a sensor isshown in FIG. 12, this cross-section corresponding, in turn, to FIG. 7.The same objects are again denoted by identical reference numerals.However, in contrast to FIG. 7, no connection region (20), which isisolated by trenches (9) from layer (6), is provided. The electricalcontacting follows in that two connection diffusions (64), which extendto a buried layer (62), are provided underneath metallization (7). Theburied layer (62) is then connected to conducting layer (3) formed as aprinted conductor. The insulation from the rest of conducting layer (6)is achieved in this case by insulating diffusions (63). The substrateunderneath the insulating diffusions is thereby p-doped. The Si-layer(6), connection diffusions (64), and buried layer (62) are then n-doped.This structure can be used quite advantageously, when the intention isto integrate a circuit together with the sensor element.

What is claimed is:
 1. A method for manufacturing a sensor, comprisingthe steps of:sequentially depositing on a substrate, a first insulatinglayer, a conducting layer directly in contact with said first insulatinglayer, a second insulating layer directly in contact with saidconducting layer, and a silicon layer directly in contact with saidsecond insulating layer; patterning the conducting layer and the secondinsulating layer before deposition of a subsequent layer; introducingtrenches into the silicon layer, the trenches extending from a top sideof the silicon layer to the second insulating layer; and applying anetching medium through the trenches to the second insulating layer. 2.The method according to claim 1, wherein the substrate is a siliconsubstrate.
 3. The method according to claim 1, wherein the first andsecond insulating layers include silicon oxide.
 4. The method accordingto claim 1, wherein the silicon layer is deposited in an epitaxialreactor.